This invention relates to an apparatus and a method for the compensation of temperature drift in Surface Acoustic Wave (SAW) filters, and particularly but not exclusively for Intermediate Frequency (I.F) SAW filters of the sort used in television systems.
The following description relates to television systems typically including a display screen and a broadcast data receiver (BDR) connected to or integrally formed therewith. The BDR receives digital data from a broadcaster at a remote location via cable, satellite and/or terrestrial broadcast systems and processes and decodes the data to provide video, audio and/or auxiliary data relating to a number of channels and/or television programs, to the user via the display screen and speakers.
In the abovementioned television systems, SAW filters are used to filter out unwanted signals that could interfere with the processing of the wanted/required signals. In digital applications, unwanted adjacent signals, such as adjacent channel signals, can sometimes be substantially greater than the required digital signals as is the case with analogue signals. To prevent adjacent unwanted signals creating interference with the required signals, the SAW filters are designed to have sharp frequency cut off characteristics. However, a problem with SAW filters is that the centre frequency of the filter drifts with changes in temperature.
Analogue television demodulators which follow the frequency of the SAW filters have to compensate for this frequency drift by also changing their center frequency. If the demodulators did not do this, it would typically result in I.F. flatness being degraded and may result in color loss of the video display. Conventionally, free running voltage-controlled oscillators (VCOs) are used to follow the frequency of the SAW filters, the VCOs having the same temperature drift performance as the SAW filters. However, the demodulators have to be manually tuned to the nominal center frequency in production and this is likely to be both time consuming and expensive. In addition, due to mechanical stress, temperature extremes and/or ageing, these demodulators have a tendency to off tune with time, rendering their performance outside the desired specification and, in some cases, the operation characteristics may be illegal.
More recently, fixed frequency demodulators have been developed using Phase Lock Loop (PLL) techniques which prevent the demodulator VCO from off tuning. However, these demodulators are unable to compensate for the SAW Filter Center Frequency Drift with temperature, thereby resulting in failure of IF flatness. This is a problem facing manufacturers of components for television systems, and particularly so for manufacturers who are required to meet US Cable Federal Communications Commission (FCC) compliance, where IF flatness is a legal requirement of analogue television demodulation.